FROM GENE TO PROTEIN. One Gene One Enzyme Hypothesis 3/12/2013. Basic Principles of Transcription & Translation

Transcription

1 One Gene One Enzyme Hypothesis FROM GENE TO PROTEIN C H A P T E R 1 7 Archibald Garrod 1 st to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions ; alkaptonuria Beadle & Tatum bread mold, Neurospora crassa; isolated mutants strains requiring arginine; concluded that each strain was defective in a single gene (see textbook for full explanation of experiment) Revisions: One Gene One polypeptide Hypothesis Not all proteins are enzymes (keratin, insulin) Some proteins constructed of more than one polypeptide chain (hemoglobin) Basic Principles of Transcription & Translation Transcription synthesis of RNA under the direction of DNA (serves as a template); occurs in nucleus of eukaryotic cells mrna messenger RNA; carries the genetic message from DNA to the ribosome Codon- sequence of mrna nucleotides that code for a specific amino acid Translation synthesis of polypeptide; cell translates base sequence of mrna into amino acid sequence ; occurs in ribosome 1

2 Transcription RNA polymerase separates DNA double helix and adds RNA nucleotides in the 5 to 3 direction; does not need a primer RNA polymerase I,II,III RNA pol II one used to for mrna sequences that are translated into proteins specific sequences of nucleotides mark where transcription can begin promoter Promoter serves as the binding site for RNA polymerase & which DNA strand serves as the template Stages of Transcription 1. Binding & Initiation Group of proteins plus RNA polymerase II bind to the promoter region of DNA forming the initiation complex Promoter commonly contains a TATA box nucleotide sequence about 25 nucleotides upstream from the start of transcription 2. Elongation RNA polymerase moves along the DNA, continuing to untwist the double helix RNA nucleotides added to 3 end of growing chain As complex moves down the DNA, the double helix reforms, while new RNA molecule pulls away from the DNA template 3. Termination Polyadenylation signal sequence (AAUAAA) is formed Polymerase continues transcribing past this sequence (100 s of nucleotides) Polymerase eventually falls off 2

3 Modification of mrna 5 cap and Poly A tail added to pre mrna protect against degradation in cytoplasm Introns noncoding regions interspersed within the coding regions Exons coding regions Introns are spliced out and exons are joined together Splicing is done by small nuclear ribonucleoproteins (snrnps) snrnps join together to form operating called spliceosomes. Translation Message is a series of codons along the mrna, the interpreter is the transfer RNA or trna trna transfer amino acids from the cytoplasm to the ribosome Cell cytoplasm stocked with all 20 amino acids trna has an amino acid at one end and a specific sequence of nucleotides at the other called the anticodon Aminoacyl-tRNA synthetase enzyme that binds the correct amino acid to the trna Ribosomes Divided into 2 subunits large & small Composed of proteins & ribosomal RNA rrna Made in nucleolus of eukaryotes Structure aids the bringing together of mrna codon & trna anticodon Ribosome has binding site for mrna Also binding sites for trna P site (peptidyl-trna site) holds trna carrying growing polypeptide chain A site (aminoacyl-trna site) holds the trna carrying the next amino acid to be added E site (exit site) discharges the trna from ribosome 3

4 Building a Polypeptide 3 steps initiation, elongation & termination Requires protein factors & energy (hydrolysis of GTP guanosine triphosphate similar to ATP) Initiation 1. Small ribosomal subunit binds to mrna searching for AUG 2. A trna having the anticodon UAC (initiator trna) pairs to the start codon AUG carries the amino acid methionine 3. Attachment of the large ribosomal subunit occurs Proteins called initiator factors bring all components together Initiator trna is in P site Vacant A site ready for next amino acid Elongation a.a. added one by one ; involves the participation of proteins called elongation factors ; 3 step cycle 1. Aminoacyl-tRNA base pairs with the mrna codon in the A site ; requires energy from hydrolysis of GTP 2. RNA molecule of large subunit catalyzes the formation of a peptide bond (between carboxyl of one a.a. & amine of another) 3. Translocation ribosome translocates the trna in the A site to the P site ; emptying trna in P site is moved to the E site then removed ; mrna moves along with its bound trna s,bringing the next codon to be translated into the A site 4

5 Termination 1. When a ribosome reaches a stop codon (UAA, UAG,or UGA) on mrna, the A site of the ribosome accepts a protein called release factor instead of trna 2. Release factor hydrolyzes the bond between the trna in the P site and the last amino acid of the polypeptide chain; polypeptide is freed from the ribosome 3. Two ribosomal subunits & all other components dissociate Protein Synthesis Summary Point Mutations & Protein Structure & Function Mutations changes in genetic information of a cell Point mutations chemical changes in just one base pair of a gene If in gamete, it can be transmitted to offspring & future generations Types of Point Mutations Base Pair Substitution replaces one nucleotide & its complementary partner with another nucleotide Silent mutations due to redundancy of genetic code, these have no effect on the protein (CCG mutated to CCA still codes for glycine) Missense mutations altered codon still codes for an amino acid & makes sense but not the right sense Nonsense mutations substitution inserts a stop codon prematurely resulting a polypeptide that is too short 5

6 Types of Point Mutations Insertions & Deletions Additions or losses of nucleotide pairs More serious effects on proteins Frameshift mutation occurs when an insertion or deletion causes the improper grouping of nucleotides in a codon scribe/ s.swf ter15/animations.html nscription.html 6